1. Field of the Invention
The present invention relates to a method of detecting an electrical angle in a synchronous motor in a sensor-free manner and regulating electric currents flowing through polyphase windings of the synchronous motor, so as to control operation of the synchronous motor. The present invention also pertains to an apparatus for the same.
2. Description of the Related Art
A motor control apparatus for controlling operation of a synchronous motor generally has a unit that detects an electrical angle of a rotor, and regulates the electric currents flowing through polyphase windings according to the observed electrical angle of the rotor. Resolvers, encoders, and other sensors may be applicable for the electrical angle detection unit. Sensor-free structures that detect the electrical angle based on the electric currents flowing through the windings have, however, proposed recently, in order to enhance the reliability of detection.
The applicants of the present invention have proposed a device that detects an electrical angle of a synchronous motor at a high accuracy even when a rotor is at a stop or rotates at a low revolving speed (JAPANESE PATENT LAID-OPEN GAZETTE No. 7-177788). The proposed device measures behaviors of electric currents flowing through the windings and detects the inductance of the windings, which is affected by the position of the rotor, and thereby the electrical angle according to the observed behaviors of the electric currents. This device is based on the finding that detection of the behaviors of electric currents flowing through any two phases leads to unequivocal determination of the electrical angle in the case of a three-phase synchronous motor. This technique is specifically effective when the rotor rotates at a low revolving speed. The applicants of the present invention accordingly have proposed a structure that detects the electrical angle according to the behaviors of electric currents flowing through the respective phase coils while the rotor is at a stop or rotates at a revolving speed of less than a predetermined level. The structure adopts the conventional method of detection utilizing the counterelectromotive forces when the revolving speed of the rotor is not less than the predetermined level. This structure thus enables the electrical angle to be detected at a high accuracy, irrespective of the rotation of the rotor, that is, whether the rotor is at a stop, rotates at a low revolving speed, or rotates at a high revolving speed.
The proposed electrical angle detection device, which detects the electrical angle based on the behaviors of electric currents, does not require a sensor, such as a resolver, but enables the electrical angle to be detected at a high accuracy even when the rotor rotates at a low revolving speed. When the electrical angle detection device is actually incorporated in a motor control apparatus that controls operation of a synchronous motor, however, problems discussed below arise in some applications of the synchronous motor. In the normal application of the synchronous motor, operation of the synchronous motor starts from the state in which the rotor is at a stop. It can thus be assumed that measurement of the electrical angle starts from the state in which no electromotive forces are generated in the respective windings of the synchronous motor. In case that the synchronous motor is attached to, for example, a drive shaft of a vehicle, the rotor may rotate with a rotation of the drive shaft, although the synchronous motor is not specifically used either as a motor or a generator. In such cases, the electromotive forces due to the rotation of the rotor cause electric currents flow through the respective phase coils. Under these conditions, a voltage is applied between the phase coils and the behaviors of electric currents are measured. The electrical angle determined from the observed behaviors of electric currents may accordingly not represent the actual electrical angle of the rotor accurately.